I'm using the PWM pin's in order to control the flowrate, and i wasn't able to find out on the internet whether the relation between PWM and flowrate is linear or not.

For now, i'm assuming this : flowrate range is 0.100 to 11mL/min, so :analogWrite(PUMP_1, 255); //leads to a maximum flow rateanalogWrite(PUMP_1, 0); // leads to the minimum flowrate which is 0.1 ml/minanalogWrite(PUMP_1, 128); // leads to half of the maximum flowrateetc etc.

Am i right or not?If not, is there a graphic i can find to do a correct conversion of what i want?

It is not linear, there is no relation and there is no graphic.It depends on the way the motor and the pump is build and the force needed and some on the PWM frequency. You have to measure the flow for the different PWM values. You could make a table with those values and let the Arduino convert the values.

With a good quality motor, the speed is more than the half at 50% PWM.With some motors they start rotation at 30% PWM.